In the late 1960s, a psychologist put twelve-month-old babies through a simple test — leave the room, come back, watch what happens — and the way each child reacted in those few seconds turned out to predict something about how they’d love people well into adulthood

That was the entire apparatus, and it has been shaping how people talk about love for half a century. Ainsworth and Bell found that the mother’s presence encouraged babies to explore the room, while her absence flattened that exploration and pushed clinging and searching upward. A sizeable minority did something stranger: they wanted contact and resisted it at the same time, arching away while gripping. Infant attachment predicted objective academic test scores more strongly than it predicted emotional and social adjustment, which is the reverse of what attachment theory had long expected. It is what the person walking back through it had been doing for the previous three hundred and sixty days.